THE CELL-ADHESION MOLECULE, CONNECTIN, AND THE DEVELOPMENT OF THE DROSOPHILA NEUROMUSCULAR SYSTEM

The connectin gene of Drosophila has been identified as a candidate di rect target of homeotic gene control and has also been implicated in t he formation of specific neuromuscular connections. The gene product, connectin, is a member of the leucine-rich repeat protein family and w e show that it is attached to the cell surface via a glycosyl-phosphat idylinositol linkage and that it can mediate homotypic cell-cell adhes ion in vitro. The expression of connectin protein during Drosophila em bryogenesis provides support for a role in adhesion in vivo. In the ce ntral nervous system, it is initially expressed on longitudinal glia a nd on a few identified neurons. These cells extend processes and conne ct up to form a continuous scaffold of connectin-expressing cells, pre saging the development of axonal pathways. Later, connectin is express ed on specific axons as they track along the connectin scaffold. Glial expression then declines and connectin appears on axons that fascicul ate with pre-existing connectin-positive bundles. Thus scaffold format ion, axon pathfinding and fasciculation involve specific contacts betw een connectin-positive cells. The timing and pattern of connectin expr ession suggest that it may play an important role in mediating specifi c interactions through homotypic cell adhesion.